When comparing Messenger vs Signal Private Messenger, the Slant community recommends Messenger for most people. In the question“What are the best messaging apps for iOS?”Messenger is ranked 5th while Signal Private Messenger is ranked 9th. The most important reason people chose Messenger is:

Pros

Pro

Uses Facebook friends to create a contacts list

Pro

Easy to share posts from Facebook

Integration with Facebook makes it easy to share any interesting posts from the Facebook timeline.

Pro

Read receipts

Can see who has seen each message in group messages.

Pro

Great UI

User interface of the application is clean, intuitive and easy to overview.

Pro

Encrypted voice calling

Unlike many other apps, signal provides not only text messaging but also live encrypted voice calling.

Pro

Truly open and secure, comes from the Open Whisper Systems Team

Backed by people nut about privacy, including

Edward Snowden, Whistleblower and privacy advocate

Laura Poitras, Oscar winning filmmaker and journalist

Bruce Schneier, internationally renowned security technologist

Matt Green, Cryptographer, Johns Hopkins University

Pro

Free as in both free beer and freedom, not as in "The customer is the product"

Pay Nothing - The development team is supported by community donations and grants. There are no advertisements, and it doesn't cost anything to use.

Cons

Con

Lack of privacy

Messenger is owned by Facebook. Based on their privacy policy, this means that any information you provide (including messages sent) are used by Facebook to profile you and sell access to your data to advertisers.

Con

Lack of end-to-end encryption

Secret (end-to-end encrypted) chats are only supported in one-to-one messages, and are not enabled by default, and the encryption has not been verified independently.

Con

No video messages

Currently there is no way to send video messages using Messenger.

Con

Centralized architecture

Signal's server architecture has been partially decentralized since December 2013, when it was announced that the messaging protocol that is used in Signal had successfully been integrated into the Android-based open-source operating system CyanogenMod. As of CyanogenMod 11.0, the client logic is contained in a system app called WhisperPush. According to Open Whisper Systems, "the Cyanogen team runs their own [Signal messaging] server for WhisperPush clients, which federates with [Open Whisper Systems' Signal server], so that both clients can exchange messages with each-other seamlessly". The WhisperPush source code is available under the GPLv3 license. In January 2016, however, the CyanogenMod team announced that they will be discontinuing WhisperPush on February 1, and recommended that its users switch to Signal.[35] After this, Signal's server architecture will be entirely centralized.